The proposed studies will investigate the cellular immune responses in murine hosts utilizing both an in vitro and an in vivo graft reaction system. The systems to be studied are C1300 neuroblastoma syngenic host combination, female hosts bearing H-Y incompatible male skin grafts and hosts bearing theta incompatible skin grafts. The systems were chosen because they are characterized by the failure to reject grafts despite the production of the humoral antibody. The cellular immune activity of various populations of lymphoid cells sensitized in vivo will be monitored by a modification of the microcytotoxicity assay which involves the prelabeling of target cells with H3 proline. The cellular immune responses will be compared with the humoral responses at various intervals after grafting. In addition, populations of lymphoid cells would be sensitized in vitro and evaluated for their ability to mediate lysis of target cells in vitro as measured by Cr51 release and to destroy grafts in vivo. The lymphocytes that are sensitized in vitro would be "fractionated" into subpopulations by depletion of complement receptor lymphocytes and theta bearing cells and evaluated for their ability to mediate lysis of the appropriate target cells.